Present telephone networks are evolving toward the almost exclusive use of digital carrier systems. At the heart of such systems are non-linear analog-to-digital (A/D) converters which encode analog voice and modem signals for digital transmission. A common D4 channel bank, for example, uses 64 kilo-bit per second (KBPS) pulse code modulation (PCM). Similarly, many systems designed for voice transmission use 32 KBPS adaptive differential PCM, or ADPCM.
It is desirable to be able to use the full capacity of a channel in transmitting data over the voice-band telephone network using analog modems. Non-linearities in such systems, however, serve as a limit on the maximum bit rate that analog modems can obtain while still maintaining a commercially acceptable error rate. This limitation is primarily due to the multiplicative effect that the non-linearities have on signals transmitted through the channel, such as PCM quantization noise, and on intermodulation distortion.
One technique and system for improving the performance of data transmission via a transmission channel in the presence of multiplicative noise is disclosed in U.S. Pat. No. 5,265,127, entitled "Non-linear Encoder and Decoder For Information Transmission Through Non-linear Channels," which is assigned to the assignee of the present invention and which is incorporated by reference herein. FIGS. 1 and 2 are, respectively, block diagrams of the modem transmitter and modem receiver disclosed in the aforementioned patent. The invention disclosed in the above-referenced patent uses a signal constellation which is arrived at by starting with a base constellation whose signal points and whose geometry are selected in accordance with conventional criteria. The base constellation is warped by adjusting the positions of its signal points according to a warp function which models the inverse of a non-linear characteristic of the transmission channel and which is known a priori. The warped signal points are then transmitted to a receiver where the inverse of the warp function is applied to the received signal points prior to processing them in a Viterbi decoder. As a result, the amount of distortion to the transmitted signal points due to the non-linear characteristic of the channel is reduced, and, in particular, the amount of distortion for points near the perimeter of the signal constellation is reduced. The Viterbi decoder can then use the standard, unmodified Viterbi decoding algorithm. One embodiment of the non-linear encoder has been incorporated into the V.34 modem standard and the V.32 Terbo standard.
The warp function typically depends upon the magnitude of the transmitted signal. Another independent factor in the warp function is a warp factor g which is selected as a function of the degree to which it is desired to warp or compress the overall base constellation prior to transmission. The desired degree of warping depends, in turn, upon the non-linear component of the transmission channel. A particular value of the warp factor g depends upon the application and may be pre-set in the transmitter and receiver based upon the expected characteristics of the channel. It is desirable, however, to be able to adapt the warp factor g to account for changes in the non-linear characteristics of the channel that may occur over time.